This invention relates to admittance sensing probes, sometimes referred to as RF probes, which are utilized to determine the condition and/or the level of materials.
Heretofore, it has been suggested that admittance and/or RF sensing probes be provided with a capacity to measure the nature of the composition as well as the level of materials so as to permit the level reading to be appropriately compensated for changes in the dielectric constant of the material whose level is being measured. Such probes are disclosed in U.S. Pat. Nos. 2,622,442--De Boisblanc et al and 3,230,770--Hermanson. As shown in the De Boisblanc et al patent, an elongated probe comprises a level measuring conductive probe electrode as well as reference or composition measuring conductive probe electrodes which are longitudinally spaced from the level probe electrodes so as to be closer to one end of the probe structure. The composition probe electrode provides the important function of compensating for variations in the dielectric constant of the materials within the vessel in which the probe has been inserted so as to permit the level measuring probe element to accurately measure the level of materials despite variations in the dielectric constant of the materials within the vessel.
While the use of the composition probe electrode does compensate for variations in the dielectric constant and does therefore improve the accuracy of the probe, the use of the composition probe electrode can produce fringing and leakage which will distort the level measurement. For example, the De Boisblanc et al patent discloses conductors leading away from the level measuring probe electrode which extends into close proximity to the level measuring probe electrodes and could produce an undesired capacitance which could distort the level and/or the composition measurement which would in turn distort the compensated level measurement. Moreover, fringing between the composition probe electrodes and the level measuring probe electrodes could occur with a similar effect. In addition to the fringing effects between the level measuring probe electrodes and the composition measuring probe electrodes, serious fringing effects can occur between the composition measuring probe electrodes and the vessel. Since it is desirable to locate the composition probe element in close proximity to the bottom of the vessel to assure that the composition probe element is always submerged, the composition probe element is necessarily located at the end of the probe structure and therefore subject to fringing with respect to the bottom of the vessel. This of course can affect the ability of the composition probe element to accurately compensate for changes in the dielectric constant of the material within the vessel.